1. Field of the Invention
This invention relates to keyboard input device and, particularly, to a keyboard input device used as an input device for personal computers.
2. Description of the Related Art
Thin keyboard input devices usually adapted to notebook type personal computers, have been proposed in a variety of kinds in which a pair of lever members are intersecting being linked together so as to serve as members for supporting the key tops and being resiliently urged upward by hollow rubber springs, in a manner that the intersecting angles of the pair of lever members that are linked together vary accompanying the ascending/descending motion of the key tops.
There has been proposed a keyboard input device in which, for example, a lever member of which the upper end is rotatably engaged with the back surface of the key top and another lever member of which the upper end is slidably engaged with the back surface of the key top, are linked together at their intersecting portions, and the key tops are supported by the pair of integrated lever members so as to be moved up and down.
Referring to FIG. 6, the switching unit of the keyboard input device of this kind has a dome-like hollow rubber spring 21, and wherein a moving contact 21b is formed at the ceiling portion 21a in the cavity of the rubber spring 21.
The rubber spring 21 is adhered to a switch board 22 with an adhesive over the whole circumference of a lower end 21d of a skirt-like outer peripheral wall 21c thereof.
The switching board 22 has a pair of fixed contacts 22a and 22a at a portion facing the moving contact 21b so as to produce the switching operation upon contacted by the moving contact 21b. 
In the switching board 22, further, a first circuit pattern 22b and a second circuit pattern 22c that are running being connected to the pair of fixed contacts 22a and 22a, are partly laminated in two layers as shown in FIG. 7.
The first circuit pattern 22b is provided with a first insulating layer 23 comprising two resist layers covering the upper portion thereof. On the first insulating layer 23 is formed a second circuit pattern 22c traversing (crossing) the first circuit pattern 22b, and on the second-circuit pattern 22c is formed a second insulating layer 24.
The first and second circuit patterns 22b and 22c which are locally formed in two layers, inclusive of the first and second insulating layers 23 and 24, are protruding having a height A from the surface of the switching board 22.
Even when the first circuit pattern 22b of a single layer is formed as shown in FIG. 8, the switching board 22 protrudes to a height B though it is smaller than the height A.
On the switching board 22 of a portion to where the rubber spring 21 is adhered, further, there is placed a lever-mounting plate 25 mounting a pair of lever members (not shown) intersecting in an X shape being linked together so as to ascend and descend.
As shown in FIGS. 7 and 8, the lever-mounting plate 25 is placed on the first and second circuit patterns 22b and 22c of the two-layer structure, or on the first circuit pattern 22b of the one-layer structure.
The switching board 22 mounting the above lever-mounting plate 25 is placed on a flat metal plate 26 and is supported in a housing that is not shown.
In the above conventional keyboard input device, when the key top (not shown) on the rubber spring 21 is pushed down, the outer peripheral wall 21c of the rubber spring 21 undergoes the elastic deformation permitting the moving contact 21b to descend.
Then, the moving contact 21b comes in contact with the pair of fixed contacts 22a to turn the switch on.
In the conventional keyboard input device in which the lever-mounting plate 25 is mounted on the insulating layers 23 and 24 that are partly protruding maintaining a height A or B above the surface of the switching board 22, however, the attitude of the lever-mounting plate 25 loses stability, the lever-mounting plate 25 rattles when the key top is depressed, and the feeling for depressing the key top is deteriorated.
Besides, since the first and second insulating layers 23 and 24 are protruding to a height of A or B, a depression of the key top causes the load of depression to be locally and repetitively exerted on the first and second insulating layers 23 and 24.
Due to the load of depression locally and repetitively exerted, therefore, cracks develop in the first and second insulating layers 23 and 24, whereby the second circuit pattern 22c formed on the first insulating layer 23 is broken and the breakdown voltage characteristics are deteriorated.
Or, water infiltrates through the cracks causing the first and second circuit patterns 22b and 22c to be corroded and broken.
This invention was accomplished in view of the above problems, and has an object of providing a keyboard input device of a high quality maintaining good feeling of operation when the key tops are depressed and suppressing the occurrence of cracks in the insulating layers on the circuit patterns.
As a first means for solving the above problem, a keyboard input device of the invention comprises a pair of lever members rotatably coupled at their intersecting portions, a lever-mounting plate for supporting the lower ends of the pair of lever members in an engaged manner, a circuit board on which the lever-mounting plate is placed and is forming a circuit pattern on a surface facing the lever-mounting plate, a hollow rubber spring mounted on the circuit board, and a key top supported by the pair of lever members and is urged by the elastic urging force of the rubber spring in a direction to separate away from the circuit board, wherein the circuit board forms an insulating layer for insulating and covering the circuit pattern, the insulating layer having a surface area larger than that of the lever-mounting plate permitting the lever-mounting plate to be placed thereon.
The insulating layer is constituted by at least two or more resist layers formed being laminated on the circuit board inclusive of the circuit pattern.
The insulating layer forms a protruded portion by causing the resist layer to be protruded on the circuit pattern, a dummy layer of a size equal to the height of the protruded portion is formed on the slope portions around the protruded portion, and an uppermost layer comprising the resist layer is laminated on the dummy layer and on the protruded portion.